The present invention relates to the field of radio communication systems.
Digital portable and mobile radios may be arranged either to communicate with one another via base stations, or directly with one another in xe2x80x98direct modexe2x80x99. The communication will typically either be over a digital simplex communication channel, or over a digital semi-duplex communication channel.
FIG. 1 illustrates the general scheme of a personal mobile radio (PMR) system 10. Portable radios 2, 4 and 6 of FIG. 1 can communicate with a base station 8. Radios 2, 4 and 6 could equally well be mobile radios mounted in vehicles. Each of the radios shown in FIG. 1 can communicate through base station 8 with one or more other radios. If radios 2, 4 and 6 are capable of direct mode operation, then they may communicate directly with one another or with other radios, without the communication link passing through base station 8.
Portable and mobile radios may employ a regular payload framing structure. This structure consists of a structure of fixed time points at which parts of the communication begin and end. Such a payload framing structure may have embedded xe2x80x98transmit interruptxe2x80x99 frames. Transmit interrupt frames are frames during which a transmitting radio can receive a signal from another radio. These frames thus provide a mechanism for signalling to or interrupting a transmitting radio unit, for example for power control.
In order to maximise the payload data rate, the embedded transmit interrupt frames need to be kept short with respect to the overall length of the framing structure. The problem with prior art transmit interrupt arrangements is however that there are instances when lengthy return signalling can over-spill the transmit interrupt frames.
A solution to this problem would be to lengthen the transmit interrupt frames. Lengthening the duration of the transmit interrupt frames will however correspondingly reduce the amount of time for transmitting useful voice or data payload on the transmission channel.
The problem could also be solved by spreading the return signalling over several transmit interrupt frames. However, this would reduce the overall signalling efficiency and lead to slower response times.
A need exists to alleviate the problems of the prior art.